The absorption of a gas into a liquid is a key process step in a variety of gas-liquid contacting systems. Gas-liquid contactors, also known as gas-liquid reactors, can be classified into surface and volume reactors where the interfacial surface area is between the two phases is created at the liquid surface and within the bulk liquid, respectively. Examples of surface gas-liquid reactors are many and include rotating disks and liquid jet contactors. Rotating disk generators are disks (rotors) partially immersed in a liquid and exposed to a stream of gas. A thin film of liquid solution is formed on the rotor surface and is in contact with a co-current reagent gas stream. The disk is rotated to refresh the liquid reagent contact with the gas. In liquid jet contactors, a single or array of liquid jets are exposed to a stream of gas in co-current, counter-current, or perpendicular configurations. In a volume gas-liquid reactor, the gas phase is dispersed as small bubbles into the bulk liquid. The gas bubbles can be spherical or irregular in shape and are introduced into the liquid by gas spragers. The bubbles can be mechanically agitated to increase the mass transfer.
In many gas-liquid contacting systems the rate of gas transport to the liquid phase is controlled by the liquid phase mass transfer coefficient, k, the interfacial surface area, A, and the concentration gradient, ΔC, between the bulk fluid and the gas-liquid interface. A practical form for the rate of gas absorption into the liquid is then:Φ=φa=kGa(p−pi)=kLa(CL*−CL)where Φ is the rate of gas absorption per unit volume of reactor (mole/cm3s), φ is the average rate of absorption per unit interfacial area (mole/cm2s), a is the gas liquid interfacial area per unit volume (cm2/cm3, or cm−1), p and pi are the partial pressures (bar) of reagent gas in the bulk gas and at the interface, respectively, CL* is the liquid side concentration (mole/cm3) that would be in equilibrium with the existing gas phase concentration, pi, and CL (mole/cm3) is the average concentration of dissolved gas in the bulk liquid. kG and kL are gas side and liquid side mass transfer coefficients (cm/s), respectively.
There are many approaches to maximizing the mass transfer and specific surface area in gas contactor systems. The principal approaches include gas-sparger, wetted wall jet and spray or atomization. The choice of gas-liquid contactor is dependent on reaction conditions including gas/liquid flow, mass transfer and the nature of the chemical reaction. Tables 1 summarize various mass transfer performance features of some conventional gas-liquid reactors. To optimize the gas absorption rate, the parameters kL, a and (CL*−CL) must be maximized. In many gas-liquid reaction systems the solubility of the CL* is very low and control of the concentration gradient is therefore limited. Thus, the primary parameters to consider in designing an efficient gas-liquid flow reactor are mass transfer and the interfacial surface area to reactor volume ratio, which is also known as the specific surface area.
TABLE 1Comparison of Conventional Gas-Liquid Reactor PerformanceReactor Typeβ (%,gas-liquidkGvolumetric(mole/kLkLaflow ratecm2s atm)(cm2/s)a(s−1)ratio)×104×102(cm−1)×102Packed Column 2-250.03-20.4-2  0.1-3.50.04-7.0(counter-current)Bubble60-980.5-21-40.5-6  0.54-24 ReactorsSpray Columns 2-200.5-20.7-1.50.1-1  0.07-1.5Plate Column10-950.5-6 1-201-2 1.0-40(Sieve Plate)
There are various gas-liquid contacting reactors whose performance is dependent on interfacial contact area. For example, the chemical oxygen iodine laser (COIL) produces laser energy from a chemical fuel consisting of chlorine gas (Cl2) and basic hydrogen peroxide (BHP). The product of this reaction is singlet delta oxygen, which powers the COIL. The present technology uses circular jets of liquid basic hydrogen peroxide mixed with chlorine gas to produce the singlet delta oxygen. In a typical generator, the jets are on the order of 350 microns in diameter or smaller. To generate the jets, the liquid BHP is pushed under pressure through a nozzle plate containing a high density of holes. This produces a high interfacial surface area for contacting the Cl2 gas. The higher the surface area, the smaller the generator will be and the higher the yield of excited oxygen that can be delivered to the laser cavity. Smaller and more densely packed jets improve the specific surface area, but are prone to clogging and breakup. Clogging is a serious problem since the reaction between chlorine and basic hydrogen peroxide produces chlorine salts of the alkali metal hydroxide used to make the basic hydrogen peroxide. This also limits the molarity range of the basic hydrogen peroxide, which reduces singlet oxygen yield and laser power. The heaviest element of the COIL system is this chemical fuel. These problems increase the weight and decrease the efficiency of the COIL laser. Thus there exists a need for a COIL laser that has increased efficiency and lower weight than present designs.
In another example, gas-liquid contactors are also used in aerobic fermentation processes. Oxygen is one of the most important reagents in aerobic fermentation. Its solubility in aqueous solutions is low but its demand is high to sustain culture growth. Commercial fermenters (>10,000 L) use agitated bubble dispersion to generate to enhance the volumetric mass transfer coefficient kLa. The agitation helps move dissolved oxygen through the bulk fluid, breaks up bubble coalescence, and reduces the boundary layer surrounding the bubbles. The interfacial area in these systems is increased by increasing the number of bubbles in the reactor and reducing the size of the bubble diameter. However, oxygen mass transfer to the microorganism is still constrained by the relatively small interfacial surface area of the bubble and the short bubble residence times. Current sparger systems (bubble dispersion) show a relatively small volumetric mass transfer coefficient kLa (˜0.2/s) and new approach for generating maximum interfacial surface area is desired to overcome these mass transfer limitations.